| Message ID | 20191113214521.20931-1-linux@rasmusvillemoes.dk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fs/namei.c: micro-optimize acl_permission_check | expand |
ping On 13/11/2019 22.45, Rasmus Villemoes wrote: > System-installed files are usually 0755 or 0644, so in most cases, we > can avoid the binary search and the cost of pulling the cred->groups > array and in_group_p() .text into the cpu cache.
diff --git a/fs/namei.c b/fs/namei.c index 671c3c1a3425..c78757435317 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -303,7 +303,12 @@ static int acl_permission_check(struct inode *inode, int mask) return error; } - if (in_group_p(inode->i_gid)) + /* + * If the "group" and "other" permissions are the same, + * there's no point calling in_group_p() to decide which + * set to use. + */ + if ((((mode >> 3) ^ mode) & 7) && in_group_p(inode->i_gid)) mode >>= 3; }
System-installed files are usually 0755 or 0644, so in most cases, we can avoid the binary search and the cost of pulling the cred->groups array and in_group_p() .text into the cpu cache. Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> --- Ballpark numbers: For example, building a random package like util-linux with "make -j8" causes about 300000 calls/s of generic_permission, with root-owned files (binaries, shared libraries, system headers, and walking the directories from / to those) outnumbering the user-owned files about 10:1, so in that case one avoids, say, 250000 calls/s of in_group_p(). Assuming that the net saving is about 20 instructions, that's 5M insn/s, which is of course too small to measure (it's in the .1% range), but might still be enough to justify this. fs/namei.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)